In recent years, along with an increase in demand of light weight and high output in the field of e.g., automobile, there has been a preference for high stress designing of a valve spring, a suspension spring, a clutch spring, and the like that are used for an engine and a suspension. Therefore, springs with excellent anti-fatigue property and anti-settling property have been strongly demanded in order to deal with an increase in applied stress. Particularly, since there has been a considerably strong demand for the increase in fatigue strength of the valve spring, there is a difficulty in meeting such demand even by employing SWOSC-V (JIS G 3566) that is said to have high fatigue strength among conventional steels.
In the spring steel that is required to have high fatigue strength, it is necessary to reduce hard non-metallic inclusions present in steel materials as small as possible. From such a view point, a high-cleanliness steel that is reduced in non-metallic inclusions as small as possible is generally used as a steel material for the above-described usage. Since the risk of wire breakage and fatigue fracture caused by the non-metallic inclusions becomes increased along with an increase in strength of the material, a demand for a reduction and downsizing of the non-metallic inclusions that are the main cause of such wire breakage and fatigue fracture has become markedly severer.
From the view point of achieving the reduction and downsizing of the hard non-metallic inclusions in a steel, various technologies have been proposed so far. For example, in Patent Publication 1, a high-cleanliness steel that is improved in cold processability and fatigue characteristics was realized by defining a composition of a non-metallic inclusion in order that the non-metallic inclusion is drawn well during hot rolling and finely dispersed by pulverization in a cold drawing or wire drawing step. Further, Patent Publication 2 defines a composition of a non-metallic inclusion that is easily drawn and formed into fine particles by hot rolling, cold rolling, and wire drawing.
On the other hand, Patent Publication 3 discloses a technology in which a melting point of an oxide-based inclusion is maintained to 1500° C. so that the inclusion becomes a low melting point composition that is easily drawn during hot rolling and cold rolling.
Patent Publication 4 defines a low melting point inclusion that is easily drawn during hot rolling and cold rolling, which is obtained by causing an Al2O3—SiO—MnO system to include MgO and/or CaO.
Further, focusing on a dissolved component in a steel in view of the fact that it is not always possible to prevent fatigue breakage only with the control of the conventional inclusion composition, Patent Publication 5 discloses a technology of improving fatigue characteristics by defining solute amounts of Al, Ca, and the like in a steel.
Patent Publication 1: JP-A-62-99436
Patent Publication 2: JP-A-62-99437
Patent Publication 3: JP-A-5-320827
Patent Publication 4: JP-A-63-140068
Patent Publication 5: JP-A-9-310145